1. Field of the Invention
The present invention relates to a method and apparatus for transmitting digital luminance and chrominance television signals, and in particular to such a method and apparatus wherein the pulse code modulated (PCM) codewords for each image row, which contain scan values, in accordance with a predetermined pattern either remain PCM codewords for transmission or are converted into shorter difference pulse code modulated (DPCM) codewords.
2. Description of the Prior Art
The concept of utilizing PCM and DPCM codewords for transmitting digital luminance and chrominance television signals is described, for example, in "Bit Rate Reduction To 140-Mbit/s From A Television Component Standard Requiring 216-228 Mbit/s," Wilson, Interim Report 111/81, Independent Broadcasting Authority, and is known as Hybrid DPCM.
The problem of selecting suitable coding arises in the digital transmission of television signals. If a color television signal which is processed in the studio with a 13.5 MHz clock pulse frequency and 8 bits per scan value is to be transmitted over digital channels at a data rate of 140 Mbit/s, it is necessary to reduce either the clock pulse frequency or the number of bits per image point. If the clock pulse frequency for the luminance is reduced to 11.25 MHz, in order to transmit below 140 Mbit/s it is additionally necessary to reduce the chrominance resolution in the vertical direction, which may visually impair the image quality.
By using DPCM in which each image point is coded with 6 bits, it would be possible to retain the spatial resolution, however, a disadvantage of this type of coding is the increased susceptibility to transmission disturbances and the circuit outlay necessary for such high clock pulse frequencies.
Susceptibility to transmission disturbances could be reduced by utilizing known hybrid DPCM, however, because a weighted mean of a number of scan values contained in the PCM codewords, which remain unchanged, serves as an estimated value for the formation of the DPCM codewords, the circuit outlay is not reduced.